The present invention relates to optical fiber couplers used in optical telecommunication networks to effect splitting/combining or multiplexing/demultiplexing of transmitted light.
A typical optical coupler of this type has heretofore been produced by locally heating a plurality of aligned optical fibers, each having only a core and a cladding with the coating peeled therefrom, with a gas burner or other heat source so that the optical fibers are fused together, and then properly pulling the fused region of the optical fibers.
More specifically, the flame of a gas burner is applied to a predetermined part of a plurality of optical fibers held in alignment with each other, and in this state the gas burner is swung all around the predetermined part to heat it, thereby fusing together the optical fibers. Then, the fused region of the optical fibers is drawn by pulling both ends of the fused region away from each other until a necessary coupling ratio is obtained, thereby forming a tapered fused region.
In the fused region, the diameter of the core of each optical fiber, which serves as an optical waveguide, decreases, and the mode field diameter of light propagating increases. In addition, the distance between the cores decreases. Consequently, coupling occurs between the cores. In other words, sinusoidal periodical coupling of light propagating takes place. This may be expressed simply by a sinusoidal function by a mode coupling equation.
The above-described optical coupler suffers, however, from the following problems:
1 When the optical coupler is produced as described above, the tapered fused region increases in length. Consequently, the central portion of the fused region has an approximately straight-line form. In this case, a coupling ratio close to that of complete coupling can be obtained. PA1 2 Optical fibers that have a small core diameter in comparison to the cladding diameter as in the case of single-mode fibers need to increase the length of the fused region in order to attain the core diameter and intercore distance required to initiate coupling, resulting in a reduction in the diameter of the tapered fused region. If the fused region becomes long in length and small in diameter, the strength of the fused region weakens. Therefore, the reliability with respect to fracture is not satisfactory. Further, if the fused region increases in length, the overall size of the optical coupler also increases. In addition, if the fused region is long, a correspondingly long time is required for fusing and pulling the optical fibers, resulting in an increase in the time needed for the production of the optical coupler. PA1 X: an axis X parallel to the optical axis in a coordinate system (X, Y) with the central point of the fused region as the origin; PA1 Y: an axis Y perpendicular to the optical axis in the coordinate system (X, Y) with the central point of the fused region as the origin; PA1 a: the value on the Y-axis of the largest-diameter portion at each end of the fused region; PA1 b: a difference between the value on the Y-axis of the largest-diameter portion of the fused region and the value on the Y-axis of the smallest-diameter portion in the center of the fused region; PA1 c: the length of 1/4 of the overall length of the fused region.
The complete coupling between optical fibers is essential characteristics for a wavelength-division multiplexer/demultiplexer, but it is a drawback to ordinary optical couplers because the coupling ratio changes with wavelength fluctuation. Therefore, this type of optical coupler has heretofore been impossible to use in wavelength-division multiplexing.
In the meantime, there is another type of optical coupler having reduced wavelength dependence, in which the coupling ratio changes only slightly with wavelength fluctuation. This optical coupler is produced as follows: In advance of the fusing process, one optical fiber alone is pulled (pre-pulling). Then, this optical fiber and another optical fiber (not subjected to pre-pulling) are held in alignment with each other, fused together, and subjected to pulling. This optical coupler makes use of the nature that the two optical fibers are different from each other in propagation constant because of the difference in the core diameter and hence incomplete coupling is realized between the optical fibers to achieve reduced wavelength dependence. However, this optical coupler needs the pre-pulling process. In addition, it involves the problem that the strength deteriorates partly because one optical fiber is heated twice and partly because the resulting fused region is smaller in diameter than the fused regions of the ordinary optical couplers.
There are other methods for reducing the wavelength dependence of optical couplers: one in which optical fibers which are varied in refractive index or other properties are coupled together, and another in which optical fibers which have different core diameters are coupled together. However, either of the methods suffers from the problem that since the optical fibers fused together are different in characteristics, when they are connected to other optical elements, mismatch occurs.